Punch, powder pressing apparatus and powder pressing method

ABSTRACT

A powder pressing apparatus comprises a die having a through hole, an upper punch and a lower punch. At least one of the upper and lower punches has a punching surface having an edge portion provided with a projection. The projection has a tip chamfered by a width not greater than 0.5 mm. The punching surface has a slope having a surface roughness Ra not greater than 1.0 μm. A rare-earth alloy powder is fed into a cavity formed in the through hole of the die. The rare-earth alloy powder in the cavity is oriented by magnetic field, and pressed by using the upper and lower punches. The upper punch and the lower punch are brought closest to each other at a minimum distance not smaller than 1.7 mm during the pressing. An obtained compact is used for manufacture of a sintered body and a voice coil motor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates for example to a punch for pressinga powder such as a rare-earth alloy powder into a compact, formanufacture of a magnet used in a voice coil motor, a powder pressingapparatus and a powder pressing method using the above punch, a compactformed by such a pressing method as above, a sintered body and a voicecoil motor using the sintered body.

[0003] 2. Description of the Related Art

[0004] Referring to FIG. 14 and FIG. 15, description will cover aconventional method for pressing the rare-earth alloy powder into acompact 8 (see FIG. 16). The compact 8 is sintered and then used as amagnet for the voice coil motor and so on.

[0005] In order to manufacture the compact 8, a tolling 1 for pressingoperation as shown in FIG. 14 is used. The tolling 1 includes a die 3having a through hole 2, a lower punch 4 to be inserted into the throughhole 2 in advance, and an upper punch 5 to be inserted into the throughhole 2. The lower punch 4 is provided with an upper surface having acenter portion formed with a generally arcuate projection 4 a and twoedge portions each formed with a flange-like projections 4 b. The upperpunch 5 has a lower surface formed with a recess 5 a. Each of the lowerpunch 4 and the upper punch 5 is made of a hard alloy such as cementedcarbide, and for protection from chipping and cracking, each tip of theprojection 4 b and an edge portion 5 b is chamfered by 0.8 mm.

[0006] When pressing, first, the lower punch 4 is lowered to form acavity 6 in the through hole 2, and the cavity 6 is fed with arare-earth alloy powder 7.

[0007] Then, the rare-earth alloy powder 7 in the cavity 6 is pressedbetween the lower punch 4 and the upper punch 5 while being oriented bya magnetic field. The pressing operation to the rare-earth alloy powder7 is continued until the two edge portions 5 b of the upper punch 5 areabout to contact the corresponding projections 4 b of the lower punch 4(until a gap between the punches becomes about 1 mm for example) asshown in FIG. 15 in order to form a shape as close as of a finalproduct.

[0008] As a result, a compact 8 as shown in FIG. 16 is obtained.

[0009] The compact 8 is formed to have a generally arcuate section,including an upper surface 8 a formed by the recess 5 a of the upperpunch 5, a lower surface 8 b formed by the projection 4 a of the lowerpunch 4, slopes 8 c formed by the projection 4 b of the lower punch 4,and end surfaces 8 d formed by a wall of the through hole 2.

[0010] As shown in FIG. 16, the compact 8 has a problem that cracks Adevelop along border portions 8 e between the upper surface 8 a and theslope 8 c.

[0011] Causes of the crack A will be described.

[0012] As shown in FIG. 14, when the rare-earth alloy powder 7 is fedinto the cavity 6, layers of a marking material B having a colordifferent from that of the rare-earth alloy powder 7 were inserted at apredetermined interval, and then the pressing operation was made. Then,as shown in FIG. 15, the gap between layers of the marking material Bwas found to be very narrow between the projection 4 b of the lowerpunch 4 and the edge portion 5 b of the upper punch 5. This indicatesthat the rare-earth alloy powder 7 has a much higher density between theprojection 4 b of the lower punch 4 and the corresponding edge portion 5b of the upper punch 5 than in other portions. This is presumably thatwhen being pressed, the rare-earth alloy powder having a poorflowability was sandwiched between a tip of the projection 4 b of thelower punch 4 and a tip of the edge portion 5 b of the upper punch 5,became unable to move, and was compressed into such a high density atthe border portion 8 e. Therefore, pressure exerted to the compact 8 isrelieved when the compact 8 is taken out of the through hole 2 of thedie 3, allowing a highly compressed portion such as the border portion 8e to expand more significantly, often developing cracks or fractures. Asimilar problem occurs in a sintering process.

[0013] If the rare-earth alloy powder 7 is pressed in a strong magneticfield not smaller than 0.5 MA/m oriented in a direction indicated by anarrow C (longitudinally of the projection 4 b) shown in FIG. 16, grainsof the magnetized rare-earth alloy powder 7 repel against each other,making the powder density higher in a region closer to a perimeter ofthe cavity 6 than in a center portion of the cavity 6, increasingfurther the density near the projection 4 b. Further, if the rare-earthalloy powder 7 fed into the cavity 6 is wiped flush at an upper edge ofthe cavity 6 by a lower edge of feeder box (not shown), region to bepressed by the projection 4 b is fed with an excess amount of therare-earth alloy powder 7 than needed. Since the rare-earth alloy powder7 does not have enough flowability, the density in this region after thepressing becomes higher than in other regions. Therefore, in thesecases, the compact 8 becomes more apt to develop the cracks or fracturesin the border portion 8 e when taken out of the cavity 6.

SUMMARY OF THE INVENTION

[0014] It is therefore a primary object of the present invention toprovide a punch, a powder pressing apparatus and a powder pressingmethod capable of preventing the cracks and fractures from developing inthe product, thereby improving productivity.

[0015] Another object of the present invention is to provide a compactmanufactured by the above method.

[0016] Still another object of the present invention is to provide asintered body and a voice coil motor using the sintered body.

[0017] According to an aspect of the present invention, there isprovided a punch used for pressing a rare-earth alloy powder, comprisinga punching surface for pressing the rare-earth alloy powder. Thepunching surface has an edge portion including a projection having a tipchamfered by a width not greater than 0.5 mm.

[0018] According to another aspect of the present invention, there isprovided a powder pressing apparatus comprising the above describedpunch having the tip chamfered by the width not greater than 0.5 mm, anda die having a through hole for insertion by the punch.

[0019] According to another aspect of the present invention, there isprovided a powder pressing method using the above punch having the tipchamfered by the width not greater than 0.5 mm and a die having athrough hole for insertion by the punch. The method comprises a firststep of feeding a rare-earth alloy powder into a cavity formed in thethrough hole, and a second step of pressing the rare-earth alloy powderfed into the cavity, by using the punch.

[0020] According to still another aspect of the present invention, thereis provided a compact manufactured by the above described powderpressing method.

[0021] The rare-earth alloy powder has a sharp-edged grain, and has poorflowability. Therefore, even if the rare-earth alloy powder is vibratedfor example at the time of pressing, the rare-earth alloy powder cannotmove smoothly in the cavity during the pressing, making difficult toachieve uniformity of the compact density. However, according to thepresent invention, by decreasing the width of the chamfered portion,which is a portion formed at the tip of the projection provided at theedge portion of the punching surface, to 0.5 mm or smaller, therebyreducing the amount of the rare-earth alloy powder pressed and cloggedby the edge portions, the flowability of the rare-earth alloy powderaround the edge portion is improved. Therefore, when a pressing force isapplied to the rare-earth alloy powder at the tip of the projection ofthe punching surface at the time of pressing, the rare-earth alloypowder moves to a region of a lower density along the slope of theprojection, without stagnating at the tip of the projection. Therefore,it becomes possible to obtain a compact having a uniform density, makingpossible to prevent the cracks and fractures caused by the non-uniformdensity.

[0022] According to an aspect of the present invention, there isprovided a punch used for pressing a rare-earth alloy powder, comprisinga punching surface for pressing the rare-earth alloy powder. Thepunching surface includes a projection, and the projection includes aslope having a surface roughness Ra not greater than 1.0 μm.

[0023] According to another aspect of the present invention, there isprovided a powder pressing apparatus comprising the above describedpunch provided with the slope having the surface roughness Ra notgreater than 1.0 μm and a die having a through hole for insertion by thepunch.

[0024] According to another aspect of the present invention, there isprovided a powder pressing method using the above punch provided withthe slope having the surface roughness Ra not greater than 1.0 μm and adie having a through hole for insertion by the punch. The methodcomprises a first step of feeding a rare-earth alloy powder into acavity formed in the through hole, and a second step of pressing therare-earth alloy powder fed into the cavity, by using the punch.

[0025] According to still another aspect of the present invention, thereis provided a compact manufactured by the above described powderpressing method.

[0026] According to the present invention, by making the slope roughnessRa not greater than 1.0 μm, it becomes possible to increase theflowability of the rare-earth alloy powder at the time of pressing.Therefore, the rare-earth alloy powder being in a high-density regionsuch as around the tip of the projection of the punching surface, movesalong the slope to a low-density region. As a result, it becomespossible to increase uniformity in the density of the rare-earth alloypowder within the cavity. Therefore, it becomes possible to obtain thecompact having a high uniformity of the density, making possible toprevent the cracks and fractures caused by the non-uniform density.

[0027] Preferably, the punching surface is made of an alloy steel or ahard alloy(cemented carbide). In this case, abrasion resistance of thepunching surface can be improved. Therefore, even if the chamferingwidth of the tip of the projection is not greater than 0.5 mm or thesurface roughness Ra of the punching surface is not greater than 1.0 μmfor increased flowability of the rare-earth alloy powder, the punchingsurface is virtually free from wear, making possible to keep the goodpressing.

[0028] Further, preferably, at least the projection in the punchingsurface has an HRA hardness not smaller than 75 and not greater than 93.In this case, even if the projection of the punching surface is machinedinto a sharp edge, the punch can be used for a long period of timewithout failure in the tip of the projection because of improvedtoughness.

[0029] The flowability becomes even more decreased if the rare-earthalloy powder is added with a lubricant. However, since the flowabilityof the rare-earth alloy powder can be increased according to the presentinvention, the uniformity of the powder density within the cavity can beincreased even if the lubricant is added.

[0030] If the rare-earth alloy powder is manufactured by a quenchingmethod, the flowability becomes extremely poor, since grain sizedistribution curve of the rare-earth alloy powder becomes sharp, withthe grain size confined in a small range. However, since the flowabilityof the rare-earth alloy powder can be improved according to the presentinvention, the powder density uniformity within the cavity can beimproved even if the rare-earth alloy powder is made by the quenchingmethod.

[0031] Preferably, a compact after the pressing has a density of 3.90g/cm³˜4.60 g/cm³. In this case, necessary strength of the compact can beachieved, and a rare-earth magnet having a good magnetic characteristiccan be obtained.

[0032] At the time of pressing, if the magnetic field is applied to therare-earth alloy powder in the cavity, in a direction vertical to adirection of pressing by the punch, grains of the magnetized rare-earthalloy powder repel against each other, and the powder density tends tobecome higher in an region closer to a perimeter of the cavity than in acenter region of the cavity. However, since the flowability of therare-earth alloy powder on the punching surface can be improvedaccording to the above invention, the rare-earth alloy powder in ahigh-density region moves to a low-density region. Therefore, differencein density of the rare-earth alloy powder within the cavity can bedecreased even if the rare-earth alloy powder is oriented in themagnetic field.

[0033] Further, if the rare-earth alloy powder is oriented in themagnetic field not smaller than 0.5 MA/m, the repelling force among themagnetized rare-earth alloy grains becomes larger, making the powderdensity distribution within the cavity non-uniform, making the densityhigher in a region closer to a perimeter of the cavity. However,according to the present invention, even if the magnetic orientation ismade as above, the non-uniformity of density within the cavity and theincrease in density near the perimeter of the cavity can be reduced,making possible to effectively reduce the cracks and fractures.

[0034] According to another aspect of the present invention, there isprovided a powder pressing method using a tolling including a die havinga through hole, and a pair of upper and lower punches. At least one ofthe upper and lower punches has a punching surface for pressing therare-earth alloy powder, and the punching surface has an edge portionprovided with a projection. The method comprises a first step of feedingthe rare-earth alloy powder into a cavity formed in the through hole,and a second step of pressing the rare-earth alloy powder fed in thecavity, by using the upper and lower punches. In the second step, theupper punch and the lower punch are brought closest to each other at aminimum distance not smaller than 1.7 mm.

[0035] According to the present invention, the compact after thepressing can have a side surface of a width not smaller than 1.7 mm.Therefore, the cracks and fractures in the side surface of the compactcan be reduced.

[0036] According to another aspect of the present invention, there isprovided a compact made of a rare-earth alloy powder, comprising: a mainsurface formed convex; another main surface formed concave; a slope froman edge of said another main surface; and a side surface formed betweensaid main surface and the slope. According to this compact, the sidesurface has a width not smaller than 1.7 mm. Or, said main surface has amaximum height H, the side surface has a width S, and S/H is not smallerthan 0.15.

[0037] According to the present invention, the width of the side surfaceof the compact is not smaller than 1.7 mm. Or, the width of the sidesurface of the compact has a ratio not smaller than 0.15 to the heightto the highest portion of said main surface. Therefore, it becomespossible to prevent extreme increase in the density in the side surface,making possible to reduce the difference in the density from otherportions. As a result, the crack and fracture in the side surface of thecompact can be reduced.

[0038] According to another aspect of the present invention, there isprovided a sintered body made of a rare-earth alloy powder, comprising:a main surface formed convex; another main surface formed concave; aslope from an edge of said another main surface; and a side surfaceformed between said main surface and the slope. According to thissintered body, the side surface has a width not smaller than 1.45 mm.Or, said main surface has a maximum height H, the side surface has awidth S, and S/H is not smaller than 0.15.

[0039] In this case, the compact at the time of the pressing is formedto have a width of a side surface not smaller than 1.7 mm. Or, the widthof the side surface of the compact at the time of the pressing is formedto have a ratio not smaller than 0.15 to the height to the highestportion of said main surface. Therefore, the cracks and fractures in theside surface of the compact can be reduced. As a result, yield in themanufacturing process can be improved, and productivity of the sinteredbody can be improved.

[0040] According to another aspect of the present invention, there isprovided a voice coil motor using the above sintered body.

[0041] The sintered body obtained as described above is less apt todevelop such a failure as cracks and fractures. Therefore, by using sucha sintered body, a voice coil motor of a stable quality can be obtained.

[0042] The above objects, other objects, characteristics, aspects andadvantages of the present invention will become clearer from thefollowing description of embodiments to be presented with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043]FIG. 1 is a schematic diagram showing a powder pressing apparatusas an embodiment of the present invention;

[0044]FIG. 2 is a perspective view showing a primary portion of upperand lower punches used in the embodiment in FIG. 1;

[0045]FIG. 3 is a side view showing a primary portion of a projection ina punching surface of the lower punch;

[0046]FIG. 4 is a schematic diagram showing a sequence of operation ofthe embodiment shown in FIG. 1;

[0047]FIG. 5 is a schematic diagram showing a primary portion fordescription of a state of compression of a powder in a tolling;

[0048]FIG. 6 is a perspective view showing a compact;

[0049]FIG. 7 is a graph showing relationships between a chamfering widthh and a rate of crack development;

[0050]FIG. 8 is a graph showing relationships between a surfaceroughness Ra and the rate of crack development;

[0051]FIG. 9 is a side view showing a primary portion of the projectionin the punching surface of the lower punch as a variation;

[0052]FIG. 10 is a sectional side view showing a primary portion of theupper and lower punches brought closest to each other according toanother embodiment of the present invention;

[0053]FIG. 11 is a graph showing relationships between a dimensionalratio S/H and the number of cracks;

[0054]FIG. 12 is a graph showing an example of relationships between awidth S of the side surface and the number of cracks;

[0055]FIG. 13 is a graph showing another example of relationshipsbetween the width S of the side surface and the number of cracks;

[0056]FIG. 14 is sectional side view of a primary portion showing astate in the tolling before pressing according to a prior art;

[0057]FIG. 15 is a sectional side view showing a primary portion of theupper and lower punches brought closest to each other according to theprior art; and

[0058]FIG. 16 is a perspective view showing a prior art compact.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0059] Embodiments of the present invention will be described here belowwith reference to the attached drawings.

[0060] Referring now to FIG. 1, a powder pressing apparatus 10 as anembodiment of the present invention comprises a cage-like frame 12. Ateach of a lower and an upper portions inside the frame 12, there isdisposed a punch fixing table 14 and a plate 16 in a horizontaldirection.

[0061] Further, a tolling 18 for pressing is provided in the frame 12.The tolling 18 is used for forming a compact 88 (to be described later)for a rare-earth magnet used in a voice coil motor for example.

[0062] The tolling 18 includes a die 22 having a through hole 20, alower punch 24 to be inserted into the through hole 20 in advance, andan upper punch 26 for insertion into the through hole 20. With thisarrangement, a cavity 27 is formed in the through hole 20 of the die 22.

[0063] The die 22 is set to die setters 28, 30. The die setter 30 has anupper surface disposed with a feeder box 32. The feeder box 32 holds arare-earth alloy powder 34. The feeder box 32 is linked to a hydrauliccylinder 38 via a cylinder rod 36. Therefore, the feeder box 32 ismovable to and from the through hole 20 by the hydraulic cylinder 38.

[0064] Each of the die setters 28, 30 has a lower surface attached witha die setter connecting plate 42 via a guide post 40. The die setterconnecting plate 42 is connected to a lower hydraulic cylinder 46 via acylinder rod 44. Therefore, the die 22 and die setters 28,30 are movablein the vertical directions by the lower hydraulic cylinder 46. An amountof extension of the cylinder rod 44, i.e. a position of the die 22 ismeasured by a linear scale 48, and the operation of the lower hydrauliccylinder 46 is controlled based on the measurement.

[0065] The lower punch 24 is provided on a base plate 50. The base plate50 is disposed on the punch fixing table 14 via a post 52. The lowerpunch 24 is fixed with this arrangement.

[0066] The upper punch 26 has an upper end attached to an upper punchplate 54. The upper punch plate 54 is connected to an upper hydrauliccylinder 58 via a cylinder rod 56. The upper hydraulic cylinder 58 isdisposed on the plate 16. Further, the upper punch plate 54 has two edgeportions each penetrated by a guide post 60. Therefore, the upper punchplate 54 is movable vertically by the upper hydraulic cylinder 58 whilebeing guided by the guide posts 60. An amount of extension of the upperpunch plate 54, i.e. a position of the upper punch 26 is measured by alinear scale 62, and the operation of the upper hydraulic cylinder 58 iscontrolled based on the measurement.

[0067] Further, in order to orient the powder 34 fed in the cavity 27 bya magnetic field, a pair of pole pieces 64, and coils 66 wound aroundpole pieces 64 respectively are provided near the die 22.

[0068] In the powder pressing apparatus 10 as described above, attentionshould be paid to the upper punch 26 and the lower punch 24.

[0069] Each of the upper punch 26 and the lower punch 24 is made of aWC-Ni type of sintered hard alloy(cemented carbide) having an HRAhardness not smaller than 75 and not greater than 93, comprisingtypically 1.6 wt % Mo and 20 wt % Ni, with the rest being WC, forexample. The term sintered hard alloy used herein refers to an alloymade by sintering the following powder mix. The powder mix comprises apowder made of a carbide comprising at least one element among nineelements belonging to groups IVa, Va, VIa of the periodic table ofelements, and a powder of ferrous metal such as Fe, Co, Ni, Mo, and Snor of an alloy made therefrom. The sintered hard alloy may alternativelybe a WC-TaC-Co alloy, a WC-TiC-Co alloy or a WC-TiC-Tac-Co alloy.

[0070] Alternatively, the upper punch 26 and the lower punch 24 may bemade of an alloy steel. The term alloy steel used herein refers to analloy primarily made of Fe-C, and may be a high speed steel, a highmanganese steel or a die steel and so on, if provided with the specifiedhardness.

[0071] By making the upper punch 26 and the lower punch 24 of thesintered hard alloy or an alloy steel having the HRA hardness of notsmaller than 75 and not greater than 93, the toughness and a certainlevel of elasticity is achieved, and it becomes possible to preventcracking and chipping even if the upper punch 26 and the lower punch 24are machined to have a sharp portion.

[0072] Referring now to FIG. 2, the lower punch 24 includes a lowerpunch main body 68. The lower punch main body 68 has an upper end formedwith the punching surface 70 for pressing the powder 34. The punchingsurface 70 includes a generally arcuate projection 72 along alongitudinal centerline, and two sides each formed with a longitudinalflange-like projection 74. As a result, a longitudinal groove 76 isformed between the projection 72 and the projection 74.

[0073] Referring further to FIG. 3, description will be made for a tip78 of the projection 74.

[0074] The tip 78 of the projection 74 is chamfered. A broken line 80shows a tip of the projection 74 before the chamfering operation. Bychamfering the tip indicated by the broken line 80, the curved tip 78having a radius R is formed. It should be noted here that the projection74 is formed to have a slope, with a portion above a border X1 formedinto a curved surface and a portion below formed into a flat surface.Further, the chamfering width h is defined as a minimum distance fromthe border X1 to a punch side surface 82. According to the presentembodiment, the chamfering width h is not greater than 0.5 mm. Further,more preferably, the chamfering width h is not greater than 0.1 mm. Froma view point of preventing failure, it is preferable that the chamferingwidth h is 0.02 mm˜0.05 mm.

[0075] Referring back to FIG. 2, the upper punch 26 includes an upperpunch main body 84. The upper punch main body 84 has a bottom end formedwith a concave punching surface 86 for pressing the powder 34.

[0076] Further, the punching surface 86 of the upper punch 26 and thepunching surface 70 of the lower punch 24 are treated to have a surfaceroughness Ra not greater than 1 μm.

[0077] According to the present embodiment, the surface treatment may bemade only to a part of the projection 74 of the punching surface 70 sothat the treated part has the surface roughness Ra not greater than 1μm.

[0078] It should be noted here that if the upper punch 26 and the lowerpunch 24 are made of the alloy steel, the surface treatment may includeTiN coating or diamond-like carbon (DLC) coating, thereby increasingdurability against an Nd-Fe-B alloy powder which is highly abrasive.Further, the same surface treatment improves durability if the upperpunch 26 and the lower punch 24 are made of the sintered hard alloy.

[0079] Still further, such a surface treatment as above can beapplicable if the upper and lower punches 26, 24 are constituted viceversa.

[0080] The rare-earth alloy powder used as the powder 34 is manufacturedas follows. Specifically, an ingot is made using a strip cast process asdisclosed in the U.S. Pat. No. 5,383,978 as a quenching method.

[0081] More specifically, an alloy manufactured by a known method andhaving a composition comprising 30% Nd, 1.0% B, 1.2% Dy, 0.2% Al, 0.9%Co (by weight) with the rest of ingredient being Fe and unavoidableimpurities is melted by a high-frequency melting process into a molten.The molten is maintained at 1,350° C., and then quenched on a singleroll. Cooling conditions at this time include a roll peripheral speed ofabout 1 m/s, a cooling rate of 500° C./sec, and a sub-cooling of 200° C.The above quenching process yields an ingot of flaky alloy having athickness of about 0.3 mm. It should be noted here that the cooling ratein the quenching method may be 10²° C./sec˜10⁴° C. /sec.

[0082] The obtained alloy ingot is coarsely pulverized by a hydrogenocclusion method, and then finely milled into an alloy powder having anaverage grain diameter(median diameter) of about 3.5 μmm by a jet millin a nitrogen atmosphere.

[0083] Such a rare-earth alloy powder as above is added with alubricant. In this case, fatty acid ester is used as the lubricant, anda petroleum solvent is used as a solvent for example. The fatty acidester diluted with the petroleum solvent is added by the amount of 0.3weight % (lubricant base) of the rare-earth alloy powder, and mixed,coating powder grain surface with the lubricant.

[0084] Next, operation of the powder pressing apparatus 10 will bedescribed with reference to FIG. 4.

[0085] A first sate is a state in which a previous cycle of the pressingoperation is completed. As shown in FIG. 4(a), the die 22 stays at anend of its downstroke while the upper punch 26 stays at an end of itsupstroke. Then, as shown in FIG. 4(b), the feeder box 32 is slid towardthe through hole 20. The feeder box 32 is stopped right above thethrough hole 12 as shown in FIG. 4(c). Thereafter, as shown in FIG.4(d), the die 22 begins rising to form the cavity 27 in an upper portionof the through hole 20, allowing the powder 34 held in the feeder box 32to fall into the cavity 27.

[0086] Next, as shown in FIG. 4(e), when the die 22 reaches an end ofits upstroke, the feeder box 32 is evacuated from above the cavity 27,wiping the powder 34 by a bottom edge of the feeder box 32.

[0087] Then, as shown in FIG. 4(f), the upper punch 26 is lowered intothe through hole 20 (the cavity 27). The powder 34 within the cavity 27is oriented by the magnetic field and the powder 34 is pressed by theupper punch 26 and the lower punch 24, into the compact 88.

[0088] When the pressing is complete, as shown in FIG. 4(g), the upperpunch 26 is raised while the die 22 is lowered, to take out the compact88.

[0089] Now, reference is made to FIG. 5 for description of a state inthe cavity 27 at the time of pressing shown in FIG. 4(f).

[0090] As shown in FIG. 5, the upper punch 26 and the lower punch 24 areinserted in the through hole 20 of the die 22 from above and belowrespectively. The powder 34 is pressed by the die 22, the upper punch 26and the lower punch 24. At this time, the magnetic filed having thestrength not smaller than 0.5 MA/m is applied in a direction vertical toa plane of the document in FIG. 5, i.e. in a direction vertical to thedirections of pressing by the upper and lower punches 26, 24, in alongitudinal direction of the compact 88 (longitudinally of theprojection 74) as indicated by an arrow C in FIG. 6. This increases thepowder density above the projection 74 than in a central portion of thecavity 27.

[0091] As shown by an arrow P in FIG. 5, the powder 34 above theprojection 74 moves toward the groove 76 with the downward movement ofthe upper punch 26.

[0092] At this time, the tip 78 of the projection 74, formed to have thechamfering width h of not greater than 0.5 mm, increasesfluidity(flowability) of the powder 34, allowing the powder 34 above theprojection 74 to move down smoothly along the surface of the projection74 without stagnating above the projection 74. Especially, since theprojection 74 is treated so as to have the surface roughness Ra notgreater than 1.0 μm, it becomes possible to move down the powder 34smoothly along the projection 74. Therefore, it becomes possible to makeuniform density distribution of the powder 34 especially near theprojection 74 including the tip 78. As a result, the compact 88 having auniform density can be obtained, making possible to prevent the cracksand fractures caused by non-uniform density distribution.

[0093] Therefore, even if the flowability of the rare-earth alloy powderis decreased by the addition of the lubricant, or if the flowability ofthe rare-earth alloy powder is very poor as a result of manufacture bythe quenching method, the uniformity of compact density can be improved.

[0094] Further, difference in the density of the powder 34 in the cavity27 can be reduced even if the powder 34 is oriented by the magneticfield not smaller than 0.5 MA/m.

[0095]FIG. 6 shows the compact 88 formed as described above.

[0096] The compact 88 is formed into a shape having a generally arcuatesection, and include a convex upper surface 90, a concave lower surface92, slopes 94 rising upwardly from two edges of the lower surface 92respectively, side surfaces 96 formed between respective pairs of anedge of the upper surface 90 and an edge of the slope 94, and generallyarcuate end surfaces 100.

[0097] The upper surface 90 is formed by the punching surface 86 of theupper punch 26. The lower surface 92 is formed by the projection 72 ofthe punching surface 70 of the lower punch 24. The slope 94 is formed bythe projection 74 of the punching surface 70 of the lower punch 24. Eachof the side surfaces 96 and the end surfaces 100 is formed by the wallof the through hole 20 of the die 22.

[0098] As has been understood from the above description made withreference to FIG. 5, the density distribution of the powder 34 is madeuniform around the side surface 96. Therefore, differing from aconventional compact 8 shown in FIG. 16, in which the densitydistribution is non-uniform, development of a crack A is virtuallyeliminated.

[0099] Further, by controlling the density of the compact 88 after thepressing to be 3.90 g/cm³˜4.60 g/cm³, necessary strength of the compact88 can be achieved, and a rare-earth magnet having a good magneticcharacteristic can be obtained. If the density is not greater than 3.90g/cm³, the strength of the compact 88 is too small for easy handling. Onthe other hand, if the density is not smaller than 4.60 g/cm³,compression rate of the compact 88 is too high, disturbing the magneticorientation.

[0100] Next, FIG. 7 shows a result of experiment indicatingrelationships between the chamfering width h of both punches and a rateof crack development.

[0101] In this experiment, the upper surface 90 had a width w of 52.22mm, and a height H to the highest point of the upper surface 90 was 30.2mm. The compact 88 had a thickness D of 25.04 mm, and the side surface96 had a width S of 7.55 mm. The surface roughness Ra of the projection74 of the punching surface 70 was 0.033 μm. The rare-earth alloy powderhaving an average grain diameter(median diameter) of 3.5 μm was used asthe powder 34, and the compact density after the pressing was 4.1 g/cm³.

[0102] The crack development rate when the chamfering width h wasbetween 0.05 mm˜0.50 mm averaged at 0.2%. When the chamfering width hwas 0.60 mm, the crack development rate was 0.4%, and when thechamfering width h was 0.70 mm, the crack development rate was 1.43%.When the chamfering width h was 1.0 mm, the crack development rate was2.36%.

[0103] Therefore, the chamfering width h is preferably not greater than0.50 mm. Further, since the crack development rate with the chamferingwidth h of 0.10 mm was 0.1% and the crack development rate with thechamfering width h of 0.05 mm was 0%, the chamfering width h is morepreferably not greater than 0.10 mm, and even more preferably, notgreater than 0.05 mm.

[0104] Further, FIG. 8 shows a result of experiment indicatingrelationships between the surface roughness Ra and the crack developmentrate.

[0105] In this experiment, the upper surface 90 had a width W of 52.22mm, and a height H to the highest point of the upper surface 90 was 30.2mm. The compact 88 had a thickness D of 25.04 mm, and the side surface96 had a width S of 7.55 mm. The chamfering width h of both punches was0.05 mm. The rare-earth alloy powder having an average grain diameter of3.5 μm was used as the powder 34, and the compact density after thepressing was 4.1 g/cm³.

[0106] The crack development rate when the surface roughness Ra wasbetween 0.05 μm˜0.52 μm was not greater than 0.4%. When the surfaceroughness Ra was 1.00 μm, the crack development rate was 0.8%. However,when the surface roughness Ra exceeds 1.00 μm, the crack developmentrate increases rapidly: for example, when the surface roughness Ra was2.45 μm, the crack development rate was 2.5%, and with the surfaceroughness Ra of 3.18 μm, the crack development rate was 3.5%.

[0107] Therefore, the surface roughness Ra is preferably not greaterthan 1.00 μm. Further, the surface roughness Ra is more preferably notgreater than 0.52 μm.

[0108] It should be noted here that a lower punch 24 a having aprojection 74 a in place of the projection 74 as shown in FIG. 9 may beused.

[0109] According to the projection 74 a of the lower punch 24 a, a flatportion 104 that is less steep than the flat portion 102 is formedbetween a chamfered curvy tip 78 a and a sloped flat portion 102.

[0110] In this case, a portion close to the tip of the projection 74 ais first machined to form the flat portion 104. A broken line 80 a inFIG. 9 indicates the tip of the projection 74 a after the formation ofthe above flat portion 104 and before the chamfering operation. Bychamfering the tip indicated by the broken line 80 a, the curvy tip 78 ahaving a radius R is formed.

[0111] According to the projection 74 a, a border X2 is a border betweenthe tip 78 a and the flat portion 104, whereas a border Y is a borderbetween the two flat portions 104 and 102.

[0112] The chamfering width h is a minimum distance from the border Y toa punch side surface 82 a, i.e. a minimum distance from a starting pointof chamfering to the punch side surface 82 a. According to the presentembodiment, the chamfering width h is not greater than 0.5 mm. Further,the chamfering width h is more preferably not greater than 0.2 mm.

[0113] According to the present embodiment, by providing the flatportion 104, the chamfering width h can be further decreased.

[0114] It should be noted here that connection between the flat portions102 and 104 should preferably be made smooth and curvy by chamfering aportion of the border Y. Further, a curved portion may be formed inplace of the flat portion 104.

[0115] Next, reference is made to FIG. 10 for description of anotherembodiment of the present invention.

[0116] In this embodiment, a conventional lower punch 106 having achamfering width h greater than 0.5 mm is used. Other constitutions arethe same as in the powder pressing apparatus 10.

[0117] At the time of pressing the powder 34, a minimum distance whenthe upper punch 26 and the lower punch 106 are brought closest to eachother is not smaller than 2 mm. Specifically, as shown in FIG. 10, a gapG between the edge 108 of the upper punch 26 and the projection 110 ofthe lower punch 106 when the two punches are brought closest to eachother is at least not smaller than 2 mm.

[0118] Therefore, the width S of the side surface 96 of the obtainedcompact 88 is not smaller than 2.0 mm. Further, S/H is preferably notsmaller than 0.15.

[0119] The compact88 is sintered thereafter for an hour at 1,000°C.˜1200° C. in an argon atmosphere as disclosed in the paragraph 10(4)of the U.S. Pat. No. 4,792,368,yieldingasinteredbody. The side surfacewidth of the sintered boy becomes not smaller than 1.7 mm, and at thistime, S/H of the sintered body is substantially the same as S/H of thecompact 88.

[0120] As described above, by forming the side surface 96 having thewidth S not smaller than 2 mm in the compact 88, extreme increase in thedensity in the side surface 96 can be avoided, thereby lessening thedensity difference from other portions. Thus, the cracks and fracturesin the side surface of the compact 88 can be reduced.

[0121] By using such a sintered body as described above, yield inmanufacturing process can be improved and productivity is increased inthe manufacture of the rare-earth magnet.

[0122] Further, by leaving the side surface 96 between the slope 94 andthe upper surface 90 of the compact 88 during the pressing, it becomespossible to use the side surface 96 as a reference surface in shaping,polishing and to a final step of machining.

[0123] Further, the sintered body which is less susceptible to the crackcan be obtained, and by using such a sintered body, a voice coil motorhaving a stable quality can be obtained.

[0124] The voice coil motor referred to herein is used for example in adisk drive as disclosed in FIG. 9 of the U.S. Pat. No. 5,448,437.According to the above FIG. 9, the voice coil motor is indicated by areference number 37. The compact 88 shown in FIG. 6 of the presentapplication is sintered, then sliced in parallel to the end surface 100,and surface-treated. The rare-earth magnet thus produced is used forexample as a magnet indicated by reference numbers 3, 4, 5 and 6 inFIGS. 1 and 2 of the U.S. Pat. No. 5,448, 437.

[0125] Next, reference is made to FIG. 11 and FIG. 12 for description ofresults of experiments conducted by the powder pressing apparatus usinglower punch 106.

[0126] The experiment used the rare-earth alloy powder added with theabove described lubricant made from the fatty acid ester. The rare-earthalloy powder was oriented in the magnetic field of 1.0 MA/m, and pressedto a green density of 4.0 g/cm³-4.2 g/cm³. Two compacts were made, andthe number of cracks developed on the side surfaces 96 in each of thecompacts 88 was counted and averaged. The average was given as “thenumber of cracks”.

[0127]FIG. 11 shows relationships between the dimensional ratio S/H andthe number of cracks. The experiment was made under two settings; withan angle θ(see FIG. 6) made by the upper surface 90 and the slope 94being 95 degrees and 120 degrees. The compact 88 used in the experimenthad a width W of 53.32 mm, the upper surface 90 with a curvature radiusof 37.58 mm, the lower surface 92 with a curvature radius of 17.55 mm,and the length of the compact 88 in the direction of orientation of 80mm. In this experiment, the chamfering width was 0.05 mm for each of theupper punch 26 and the lower punch 106 and the surface roughness Ra ofthe slope is 0.033 μm.

[0128] As shown in FIG. 11, there is virtually no crack development ifthe dimensional ratio S/H is not smaller than 0.15. With the dimensionalratio S/H exceeding 0.2 there was actually no crack development, exceptthe case in which the angle θ is 95 degrees.

[0129]FIG. 12 shows relationships between the width S of the sidesurface 96 and the number of cracks. The experiment was made under threesettings; with the angle θ being 90 degrees, 115 degrees and 130degrees. The compact 88 used in the experiment had the width W of 33.57mm, the upper surface 90 with a curvature radius of 20.84 mm, the lowersurface 92 with a curvature radius of 13.27 mm, and the length of thecompact 88 in the direction of orientation of 80 mm. In this experiment,the chamfering width was 0.8 mm for each of the upper punch 26 and thelower punch 106 and the surface roughness Ra of the slope is 0.033 μm.

[0130] As shown in FIG. 12, there is virtually no crack development ifthe width S is not smaller than 2 mm. With the width S not smaller than3 mm, there was actually no crack development.

[0131] Further, experiment similar to those summarized in FIG. 10 andFIG. 12 was conducted using the powder pressing apparatus 10 having thelower punch 24 with the chamfering width h being 0.05 mm. Except for theuse of the lower punch 24, the experiment was made under the sameconditions as those summarized in FIG. 10 and FIG. 12, and a resultshown in FIG. 13 was obtained.

[0132]FIG. 13 shows relationships between the width S of the sidesurface 96 and the number of cracks.

[0133] As shown in FIG. 13, there is virtually no crack development ifthe width S is not smaller than 1.7 mm. With the width S not smallerthan 3 mm, there was actually no crack development.

[0134] As has been exemplified as above, the width S can be furtherreduced if the powder pressing apparatus 10 provided with the lowerpunch 24 having the chamfering width h of not greater than 0.5 mm isused. Therefore, the minimum distance between the upper punch 26 and thelower punch 24 brought closest to each other, i.e. the gap between anedge 108 of the upper punch 26 and the projection 74 of the lower punch24 when the two punches are brought closest to each other can be madenot smaller than 1.7 mm.

[0135] For a reference, if the compact 88 having the side surface 96 ofthe width S being 1.7 mm is sintered, the width of the side surface ofthe sintered body becomes 1.45 mm.

[0136] It should be noted here that the projection 74 of the lower punch24 should be formed longitudinally of the punching surface 70, at leastpartially thereof.

[0137] Further, at least one of the upper punch and the lower punchesshould have the chamfering width h not greater than 0.05 mm.

[0138] The present invention being thus far described and illustrated indetail, it is obvious that these description and drawings only representan example of the present invention, and should not be interpreted aslimiting the invention. The spirit and scope of the present invention isonly limited by words used in the accompanied claims.

What is claimed is:
 1. A punch used for pressing a rare-earth alloypowder, comprising: a punching surface for pressing the rare-earth alloypowder; wherein the punching surface has an edge portion including aprojection having a tip chamfered by a width not greater than 0.5 mm. 2.A punch used for pressing a rare-earth alloy powder, comprising: apunching surface for pressing the rare-earth alloy powder; wherein thepunching surface includes a projection, the projection including a slopehaving a surface roughness Ra not greater than 1.0 μm.
 3. The punchaccording to claim 1 or 2, wherein the punching surface is made of analloy steel or a sintered hard alloy.
 4. The punch according to claim 1or 2, wherein at least the projection in the punching surface has an HRAhardness not smaller than 75 and not greater than
 93. 5. A powderpressing apparatus for pressing a rare-earth alloy powder, comprising: adie having a through hole; and a punch to be inserted into the throughhole; wherein the punch includes a punching surface for punching therare-earth alloy powder, and the punching surface including an edgeportion provided with a projection having a tip chamfered by a width notgreater than 0.5 mm.
 6. A powder pressing apparatus for pressing arare-earth alloy powder, comprising: a die having a through hole; and apunch to be inserted into the through hole; wherein the punch includes apunching surface for pressing the rare-earth alloy powder, the punchingsurface being provided with a projection, the projection including aslope having a surface roughness Ra not greater than 1.0 μm.
 7. A powderpressing method using a punch and a die, the punch including a punchingsurface for pressing the rare-earth alloy powder, the punching surfaceincluding an edge portion provided with a projection having a tipchamfered by a width of not greater than 0.5 mm, the die having athrough hole for insertion by the punch, the method comprising: a firststep of feeding a rare-earth alloy powder into a cavity formed in thethrough hole; and a second step of pressing the rare-earth alloy powderfed into the cavity, by using the punch.
 8. A powder pressing methodusing a punch and a die, the punch including a punching surface forpressing the rare-earth alloy powder, the punching surface including aprojection provided with a slope having a surface roughness Ra notgreater than 1.0 μm, the die including a through hole for insertion bythe punch, the method comprising: a first step of feeding a rare-earthalloy powder into a cavity formed in the through hole; and a second stepof pressing the rare-earth alloy powder fed into the cavity, by usingthe punch.
 9. The method according to claim 7 or 8, wherein therare-earth alloy powder is added with a lubricant.
 10. The methodaccording to claim 7 or 8, wherein the rare-earth alloy powder ismanufactured by a quenching method.
 11. The method according to claim 7or 8, wherein a compact after being pressed in the second step has adensity of 3.90 g/cm³˜4.60 g/cm³.
 12. The method according to claim 7 or8, wherein the second step includes applying a magnetic field fororientation of the rare-earth alloy powder in a direction vertical to adirection of the pressing by the punch.
 13. The method according toclaim 7 or 8, wherein a pair of upper and lower punches including saidpunch is used, the upper punch and the lower punch being brought closestto each other at a minimum distance not smaller than 1.7 mm in thesecond step.
 14. A powder pressing method using a tolling including adie having a through hole, and a pair of upper and lower punches, atleast one of the upper and lower punches having a punching surface forpressing the rare-earth alloy powder, the punching surface having anedge portion provided with a projection, comprising: a first step offeeding the rare-earth alloy powder into a cavity formed in the throughhole; and a second step of pressing the rare-earth alloy powder fed intothe cavity, by using the upper and lower punches; wherein the upperpunch and the lower punch being brought closest to each other at aminimum distance not smaller than 1.7 mm in the second step.
 15. Themethod according to one of claims 7, 8 and 14, wherein the second stepincludes applying a magnetic field not smaller than 0.5 MA/m fororientation of the rare-earth alloy powder.
 16. A compact manufacturedby a powder pressing method using a punch and a die, the punch includinga punching surface for pressing the rare-earth alloy powder, thepunching surface including an edge portion provided with a projectionhaving a tip chamfered by a width of not greater than 0.5 mm, the dieincluding a through hole for insertion by the punch, the methodcomprising: a first step of feeding a rare-earth alloy powder into acavity formed in the through hole; and a second step of pressing therare-earth alloy powder fed into the cavity, by using the punch.
 17. Acompact manufactured by a powder pressing method using a punch and adie, the punch including a punching surface for pressing the rare-earthalloy powder, the punching surface including a projection provided witha slope having a surface roughness Ra not greater than 1.0 μm, the dieincluding a through hole for insertion by the punch, the methodcomprising: a first step of feeding a rare-earth alloy powder into acavity formed in the through hole; and a second step of pressing therare-earth alloy powder fed into the cavity, by using the punch.
 18. Acompact made of a rare-earth alloy powder, comprising: a main surfaceformed convex; another main surface formed concave; a slope from an edgeof said another main surface; and a side surface having a width notsmaller than 1.7 mm formed between said main surface and the slope. 19.A compact made of a rare-earth alloy powder, comprising: a main surfaceformed convex; another main surface formed concave; a slope from an edgeof said another main surface; and a side surface formed between saidmain surface and the slope; wherein said main surface has a maximumheight H, the side surface having a width S, with S/H being not smallerthan 0.15.
 20. A sintered body made of a rare-earth alloy powder,comprising: a main surface formed convex; another main surface formedconcave; a slope from an edge of said another main surface; and a sidesurface having a width not smaller than 1.45 mm formed between said mainsurface and the slope.
 21. A sintered body made of a rare-earth alloypowder, comprising: a main surface formed convex; another main surfaceformed concave; a slope from an edge of said another main surface; and aside surface formed between said main surface and the slope; whereinsaid main surface has a maximum height H, the side surface having awidth S, with S/H being not smaller than 0.15.
 22. A voice coil motorusing a sintered body of a rare-earth alloy powder, the sintered bodycomprising: a main surface formed convex; another main surface formedconcave; a slope from an edge of said another main surface; and a sidesurface having a width not smaller than 1.45 mm formed between said mainsurface and the slope.
 23. A voice coil motor using a sintered body of arare-earth alloy powder, the sintered body comprising: a main surfaceformed convex; another main surface formed concave; a slope from an edgeof said another main surface; and a side surface formed between saidmain surface and the slope; said main surface having a maximum height H,the side surface having a width S, with S/H being not smaller than 0.15.